The proposed research is designed to identify and measure the neural substrates of stereo depth perception, by comparing perceptual data with functional magnetic resonance imaging (fMRI) measurements of brain activity in specific brain areas. Brain areas where the neural activity correlates with the percept may be involved in, or responsible for, stereo depth perception. Our general approach will be to replicate experiments that have yielded landmark findings in stereo psychophysics, while collecting fMRI measurements of brain activity. The fMRI data will be analyzed separately in each of several identifiable visual brain areas to link brain activity with perception, and to test psychophysically-motivated theories about how stereoscopic vision works. The specific aims of the proposed research are: (1) to measure and characterize fMRI responses to binocular stimulation in each of several brain areas, and to identify brain areas that respond to stereoscopic stimuli; (2) to compare brain activity and psychophysical upper depth (maximum disparity) limits; and (3) to compare brain activity and temporal depth modulation thresholds (rate of stereo vision processing). These experiments could improve out understanding of stereoblindness, which affects 6-7 percent of the US population.